The present invention relates to the recovery of maleic anhydride from an organic absorbent. Maleic anhydride can be produced by vapor phase oxidation of a hydrocarbon feedstock in an oxidation reactor followed by recovery and then purification of the maleic anhydride. The most common feedstocks for maleic anhydride plants include benzene, butene and butane.
Recovery of the maleic anhydride from the gaseous effluent stream from the oxidation reactor can be done by scrubbing the effluent with water, which results in conversion of the anhydride to an acid. The acid then needs to be dehydrated to produce the anhydride product. Recovery of maleic anhydride from the oxidation reactor effluent using an organic absorbent as opposed to an aqueous absorbent has also been disclosed --for example, in U.S. Pat. Nos. 2,574,644; 2,893,924; 3,040,059; British Pat. No. 727,828 and Japanese Pat. Nos. 35-7460 and 32-8408. Commonly assigned U.S. patent applications Ser. Nos. 209,069; 310,320; and 427,177 also disclose organic absorbents for maleic anhydride recovery (removal), and the disclosures of the aforesaid patents and patent applications are incorporated herein by reference, particularly in their disclosure of various organic absorbents for recovery of maleic anhydride from gas streams containing maleic anhydride.
U.S. Pat. No. 2,574,644, in particular, discloses the use of dibutylphthalate for the recovery of maleic anhydride or phthalic anhydride from an oxidation reactor effluent stream. According to the process disclosed in U.S. Pat. No. 2,574,644 the oxidation reactor effluent is cooled to first condense a portion of the anhydride vapor. The remaining gaseous stream is contacted with the dibutylphthalate absorbent to remove the remaining uncondensed anhydride by absorption into the organic absorbent. The resulting rich absorbent is stripped to obtain a product anhydride stream.
The removal of maleic anhydride or phthalic anhydride from the organic absorbent in the process disclosed in U.S. Pat. No. 2,574,644 is carried out using a "rectifier" distillation column and a "stripper" distillation column. Basically the process is a two-stage stripping operation to thoroughly strip the anhydride out of the absorbent before the absorbent is reused for absorbing anhydride out of the oxidation reactor effluent gas stream containing the anhydride. Thus, after absorption of maleic anhydride into the absorbent in the absorber column, the anhydride-rich absorbent is passed to the first main stripper column, which is called a rectifier. A vacuum is pulled by a steam jet ejector off the top of the rectifier. The anhydride-rich absorbent stream is fed to the rectifier and the anhydride is stripped out of the absorbent by heat applied to the bottom of the rectifier column. The rectifier overhead is partially condensed to obtain liquid anhydride; part of which is returned to the rectifier as reflux and the other part of which is withdrawn as product anhydride.
Stripped absorbent in the U.S. Pat. No. 2,574,644 process is withdrawn from the bottom of the rectifier and passed to the upper part of a second distillation column, which is referred to as the stripper column. The stripper column is operated using a reboiler and a separate vacuum system so that the stripper column can be operated at a lower vacuum pressure than the rectifier. Thoroughly stripped absorbent is withdrawn from the bottom of the stripper column while a portion of the partially condensed overhead from the stripper is returned to the middle of the rectifier column.